The myotonias and periodic paralyses are heritable diseases of skeletal muscle in which mutations of voltage-gated ion channels alter the electrical excitability of the sarcolemma. The long-term goals of this project are to characterize the functional defects of mutant channels and to determine how abnormal channel behavior produces symptoms. For these disorders, muscle dysfunction is caused by intermittent derangements in electrical excitability, which may be pathologically enhanced or depressed. Myotonia is a disorder of enhanced excitability wherein a single stimulus elicits a burst of action potentials that produces involuntary persistent muscle contraction lasting seconds. Conversely, periodic paralysis results from a depolarization-induced loss of muscle excitability. Chloride channel mutations cause myotonia, whereas mutations in potassium or calcium channels give rise to periodic paralysis. Missense mutations in a skeletal muscle sodium channel (NaV1.4 encoded by SCN4A) may cause myotonia, periodic paralysis, or of both in the same individual. The pathophysiological basis for this variation in clinical phenotype, all arising from mutations in a common sodium channel gene, is a major focus of the studies in this proposal. Our experimental approach is to identify alterations in the behavior of mutant channels by measuring ionic current, and then use computer or animal-based models to explore how specific alterations in channel function cause myotonia or periodic paralysis. Aim 1 is to identify the functional defects for additional, as-yet uncharacterized, NaV1.4 mutants and thereby define further the biophysical profile of gating defects that give rise to specific forms of altered membrane excitability. Prior work, by us, and others, has identified slow-inactivation gating as a critical determinant in the predisposition to attacks of weakness. In comparison to other gating transitions, relatively little is known about slow inactivation. Aim 2 seeks to improve our understanding of slow inactivation through a combination of refined voltage-dependent gating protocols and structural studies based on cysteine-scanning mutagenesis. Aim 3 explores how functional defects in Na channel behavior cause myotonia or periodic paralysis by characterizing a mouse model with a targeted mutation in NAY1.4 and using computer-based models of muscle excitability. More than 30 human disorders are known to be caused by mutations in voltage-gated ion channels. The proposed studies are designed to provide a more complete understanding of the pathophysiological basis for a group of human channelopathies: from gene defect to clinical symptoms. These studies will also further our knowledge of Na channel function at the molecular level, will lead to an improved understanding of the determinants of muscle excitability, will provide mechanistic insights for a more rational design of therapeutic strategies, and will serve as a model system for understanding more common disorders of excitability such as epilepsy or cardiac arrhythmia.